1. Field of the Invention
The invention relates to a control system for maintaining the power factor of an electrical supply line at a predetermined value. More particularly, the invention relates to a power factor control system using two cam-actuated stepping switches for adding or subtracting capacitors on or from the power supply line in a next-one-available cycle in which the first capacitor to be added is the capacitor which was removed the longest and the first capacitor to be removed is the capacitor which has been energized the longest, providing uniform usage for the capacitors and associated controls.
2. Description of the Prior Art
It is well known that a power distribution system may be operated with substantial savings if the power factor of the system is maintained close to unity. Poor power factor requires that a given load consumes more KVA than the same load would consume at a better power factor. This increase in KVA creates an increase in amperage, requiring larger wires, larger equipment such as transformers, switch gear, etc., without an increase in usable power (KW). For this reason, utility companies are permitted to charge customers an additional premium for a poor power factor.
It has become the practice for many industrial customers to install capacitors which will decrease the inductance KVAR which occurs in an electrical system primarily due to motor loads, transformers and other types of equipment which create an excessive amount of inductance resulting in a poor power factor. Common practice is to locate these capacitors near the equipment having the poor power factor, such as the motor or the transformer. If the capacitors are not placed directly in the motor circuit, the problem arises on the efficient control of the capacitors. If the capacitors remain on the power supply line at all times, it could result in a leading power factor, causing undesirable conditions such as high voltage. Therefore, many systems use time clocks or other control equipment for adding and subtracting capacitors on and from the system, as required. Since reliable capacitor controls are expensive, it is not efficient to correctly control capacitors at multiple locations. Therefore, power factor control capacitors usually are grouped at a central location and controlled by a common controller or control system.
There are various types of controllers and control systems which add and subtract capacitors on and from a power supply line in relationship to the KVAR of the supply line. Examples of such capacitor controllers and control systems are shown in U.S. Pat. Nos. 2,293,484, 3,002,146, 3,300,712 and 3,391,329. One disadvantage of known capacitor controllers and related systems, such as shown in the above-listed patents, is the manner in which the capacitors are added or subtracted from a supply line. Most known systems use a type of reversible control motor which will advance to add capacitors and will reverse to substract capacitors. This type of reversible switching will result in the capacitors being used in a last-on, first-off cycle. Accordingly, certain capacitors and their associated switching controls are used considerably more often than other capacitors and controls in the capacitor bank. This results in poor equipment usage, premature replacement, and increased maintenance.
Control systems for other types of electrical apparatus which are completely unrelated to capacitors and power factor correction have recognized this unequal equipment usage in connection with the adding and subtracting of electrical loads. Examples of these systems are described in U.S. Pat. Nos. 3,489,882, 3,529,173 and 3,787,729. These systems either require manual advancement of a rotary switch or elaborate electronic circuitry for achieving the desired uniform equipment usage.
Therefore, the need has existed for a power factor control system using electromechanical components which automatically adds and subtracts capacitors on and from an electrical supply line, which achieves uniform usage of the capacitors and associated controls by providing a last-on, last-off cycle. There is no known power factor control system which achieves this advantage by the use of two independent cam-actuated switching circuits which provide the desired switching cycle.